Gas turbines typically contain a plurality of turbine blades arranged around the circumference of a rotor. During operation of the gas turbine, expanding gasses produced in an associated turbine combustor impart force on the blades to turn the rotor. The resulting rotational energy may in turn be converted to electrical power via a generator attached to the rotor.
Some turbines contain multiple rows of freestanding blades that are not in contact or supported by adjacent blades, and these freestanding blades can be damaged by excessive vibration due to dynamic conditions such as flow induced vibration, combustion dynamics, and nozzle effects. It is therefore desirable to monitor the blade vibrations so that turbine failure can be avoided. Conventional methods for measuring the blade vibrations include the use of strain gauges attached to the blades. However, such monitoring methods are usually very complex and expensive due to the electrical connections between stationary and rotating components. Other conventional methods for measuring the vibratory response of the blades during operation include capacitance, magnetic, or optical probes mounted to the blade shroud to measure the blade tips as they pass by the probes. However, many of these blade-tip measurement techniques are of limited utility because they do not directly measure torsional and other vibrations at the blade edges.
Other methods for measuring the vibratory response of the turbine blades have been attempted, but the hostile environment within the turbine can be a difficult challenge. Relatively high temperatures within the turbine combustor often reach or exceed 1,200 degrees F. (649 Celsius), which is above the melting temperature of many sensor materials. Furthermore, combustion byproducts and exhaust can contaminate the sensor components.
For example, mirrors have been placed at the end of optical probes near the trailing edge of a row of turbine blades to direct laser light perpendicular to the surface of the turbine blade trailing edge. The light reflected by the turbine blades is then re-directed by the same mirror back to a detector that may reside outside of the hostile environment of the turbine. The mirror in this case enables directing laser light into the exhaust frame at an acute angle with respect to either vertical or horizontal direction, while the path of the light may be modified by the mirror such that it is incident perpendicular to the surface of the trailing blade edge. The light reflected from the trailing blade edge can then re-directed by the mirror along the optical path to the detector. One problem with this approach is that the turbine exhaust gas can contain unburned fuel, leftover combustion chemical species, and other debris that can cause the optical probe and the mirror to become dirty over time, thereby reducing the signal needed to determine the vibratory response of the blades. Cleaning the optical element and the mirror can be difficult in such configuration due to limited accessibility.
Therefore, a need remains for improved systems and methods for measuring turbine blade vibratory responses.